Compton scattering of polarized γ rays by¹⁶O for E _γ=25-40 MeV

Measurements of the polarization asymmetries σ(θ _c.m.) and σ(E_γ) of Compton scattering by ¹⁶O have been performed in the energy range of 25-40 MeV over a range of scattering angles between 90 and 150°. An analysis of the present data combined with previous results indicates that significant, narrow concentrations of E2 strength are not present below an excitation energy of 40 MeV. The existence, however, of a broad isovector giant quadrupole resonance exhausting a large percentage of the isovector energy weighted sum rule is not ruled out by the combined data. Additionally, the present data are insensitive to modifications to the free values of the nucleon polarizabilities, but cannot rule them out.